Refraction is the well-known phenomenon whereby light changes direction when passing from a medium having a first index of refraction to one having a second index of refraction unless the beam of light strikes the interface between the two media perpendicular to that interface. Traditional lenses operate because of refraction. Such lenses, operating by refraction, have been known for many years.
It is also possible to construct lenses operating due to the principle of diffraction. Diffractive lenses, while of more recent origin than refractive lenses, still have been known for over one hundred years. Diffractive lenses are very sensitive to the wavelength of the light striking them. Furthermore they require that the sizes of the diffractive regions be very precisely manufactured. Therefore diffractive lenses were not in significant usage until recently. With the advent of good monochrome light sources, such as lasers and light emitting diodes, and precise machining and replication techniques, diffractive lenses have become more important.
One type of diffractive lens is sometimes called a kinoform. In such a lens the diffractive zones are formed as a series of structures on a surface of the lens. These structures can be formed in a variety of ways. They may be etched using a photolithographic process, they may be directly lathe cut in the surface of the lens, or a master may be manufactured from which the lenses are manufactured using known techniques such as injection molding. Alternatively the structures may be formed at the interface of two materials with different indices of refraction in order to construct a kinoform with smooth outer surfaces. Similarly the structures may be formed by introducing a dopant that alters the index of refraction into selected regions of the lens substrate.
In order for the lens to operate properly the sizes of the zones must be very precisely controlled. Because the zone widths are smaller in the zones farther from the optical axis of the lens, it is often hard to form the outer zones. This problem is particularly pronounced in "fast" lenses, i.e. those designed to operate at a high aperture to focal distance ratio. In such lenses the outer zones are very narrow.